Polymeric materials, such as plastics, have been developed as alternatives and replacements for silica based inorganic glass in applications such as, optical lenses, fiber optics, windows and automotive, nautical and aviation transparencies. These polymeric materials can provide advantages relative to glass, including, shatter resistance, lighter weight for a given application, ease of molding and ease of dyeing. Representative examples of such polymeric materials include, poly(methyl methacrylate), polycarbonate and poly(diethylene glycol bis(allylcarbonate)).
The refractive indices of many polymeric materials are generally lower than that of high index glass. For example, the refractive index of poly(diethylene glycol bis(allylcarbonate)) is about 1.50, compared to that of high index glass, which can range, for example, from 1.60 to 1.80.
Polymeric materials (polymerizates) prepared from the polymerization of monomers containing aromatic rings and/or sulfur typically have high refractive indices. Polymeric materials having a combination of high refractive indices, such as at least 1.57, and low levels of chromatic dispersion (e.g., having ABBE numbers of at least 30), can be prepared from monomers containing certain heteroatoms, such as sulfur atoms. Such polymerizates are very useful in the making of optical elements requiring superior optical properties. However, yellowing is often a problem in polymerizates that contain sulfur. Also, polymerization initiators that are conventionally used in such compositions can have safety risks associated with them. For example, peroxides are ignitable and present explosion hazards. Certain initiators, including peroxides and azo initiators, can generate gases during polymerization, causing safety concerns and potentially affecting the appearance of the final polymerizate product.
It would be desirable to develop polymerizable compositions that provide desirable optical properties, such as high refractive index, with acceptable yellowness indices.